The present invention relates to a method for mounting a chassis to a shelf or rack of an aircraft. More particularly, the present invention relates to mounting an inertial sensor chassis to an aircraft shelf which requires precise alignment relative to the aircraft.
Inertial reference units or inertial navigation units generally include inertial sensors such as accelerometers and gyroscopes, i.e., gyros. The sensors are generally rigidly and precisely mounted to an inertial sensor base which, in turn is precisely mounted within a container, herein referred to as simply a chassis, along with related electronics and hardware. The sensor base is mounted within and to the chassis through suspension mounts or isolators. In turn, the chassis is rigidly and precisely mounted to a support frame of an aircraft. The support frame generally being referred to, in the art, as a shelf or rack of the aircraft. The precision mounting of these components is required so that the alignment of the sensor base relative to the support frame is known and the sensor outputs are utilized by a navigational system computer as is well known in the art.
As is well understood in the art, the navigational system generally includes a plurality of inertial sensors and a navigational computer. The inertial sensors provide inertial data, such as linear acceleration and rotational velocity or angular information, to the navigational computer which processes the information for either flight control and/or navigation. For proper performance of a navigational system, the geometrical relationship between each of the inertial sensors must be known, and the relationship between each of the inertial sensors and the aircraft support frame must also be known so that the navigational computer may provide a pilot with correct navigational information so that the pilot achieves the intended destination, and/or flies the aircraft "by wire".
For optimum performance of inertial reference systems or inertial navigation systems, precise alignment or orientation of the inertial sensor chassis relative to the aircraft must be known and held to tight tolerances. Commonly, the inertial sensor assembly and chassis are manufactured under optimum conditions, and therefore precise alignment of inertial sensors relative to the chassis is known. In turn, the chassis is generally delivered to the aircraft manufacturer or to an airline maintenance operation for installation. In the latter, the maintenance operation commonly will replace inertial reference units or inertial navigational units in accordance with regular maintenance schedules or as required due to system failures. This, of course, means that the inertial sensor chassis needs to be removed from the aircraft support frame and replaced with another inertial sensor chassis while maintaining proper alignment of the newly mounted chassis onto the aircraft support frame.
Accordingly, the inertial sensor chassis mounting system should provide (i) ease of installation and removal of the chassis onto the aircraft shelf, (ii) precision of alignment of the chassis relative to the aircraft shelf, (iii) repeatability of the precision alignment even with multiple repeat installations and removals, (iv) rugged support of the chassis in the aircraft shelf during acceleration loads, shock and vibration while maintaining precise alignment of the chassis during these environments, and (v) include electrical grounding and bonding of the chassis to the aircraft shelf.
In the prior art, a plurality of alignment pins and swing bolts were utilized to secure the inertial reference or navigational unit to the aircraft shelf. However, the prior art system, as will be briefly described below, resulted in imprecise yaw alignment and other deleterious effects due to the technique of swing bolts utilized to fasten the system to the aircraft shelf.
A "swing bolt" inertial sensor chassis mounting system of the prior art generally comprised the use of two rear mounted support frame alignment pins, one front mounted support frame alignment pin, three mating chassis bushings, and two front support frame hold down swing bolts. The two rear alignment pins, one round pin and one diamond shaped cylindrical pin, establish pitch alignment of the chassis relative to the aircraft support frame. A round front mounted alignment pin in combination with the rear mounted round alignment pin established both the roll and yaw alignment of the chassis relative to the support frame.
In the swing bolt mounting system, a pair of swing bolts are generally swung from the aircraft support frame and grab an ear-like member at each front corner of the chassis. In turn, a nut or bolt was threaded to force the chassis against the air frame by virtue of the force applied between the corner of the chassis and the other end of the swing bolt mounted to the aircraft support frame.
With the Swing bolt mounting .system of the prior art, pitch, roll, and yaw alignment are provided by the two rear and one front alignment pins. The swing bolts provide the feature of holding the front of the chassis down on the top of the front alignment pin, only. With use of preloaded mating bushings for the rear of the chassis, the rear of the chassis is held down on top of the rear alignment pins. Pitch and roll alignment are provided by the top surfaces of the two rear alignment pins together with the top surface of the front alignment pin; and the yaw alignment is provided by the left and right side surfaces of the front alignment pin and the one rear round alignment pin. Therefore, since the yaw alignment is provided by the position of the one rear alignment pin and the front alignment pin in the bushings of the chassis, the accuracy and repeatability of the alignment is determined by how well the pins fit in the bushings.
Of course, there is always some small clearance between the outside diameter of the alignment pins on the shelf and the inside diameter of the mating bushings on the chassis. This is so to avoid the possibility of physical interference which, of course, would prevent installation of the chassis Since there must be some clearance between the alignment pin diameter and the chassis bushing diameter as aforesaid, the resulting clearances results in uncertainty and non-repeatability of the yaw alignment in the swing bolt mounting system. On the other hand if the tolerances are held extremely small, the chassis may not be able to be installed. To repeat, if the pin-to-bushing fit is too loose, the yaw alignment is not as good; if the pin-to-bushing fit is too tight, the chassis may not be able to be installed.
Another disadvantage of the "swing bolt" inertial sensor chassis mounting system of the prior art is that swing bolt mounting applies force at an angle to the bottom plane of the chassis relative to the support frame of the aircraft. Therefore, the force from these two swing bolts can force bending of the chassis. Since the swing bolts are not usually tightened simultaneously, nor to the same force, an uneven force may be applied to the front chassis corners, which in turn may cause uneven bending of the chassis. This uneven bending or distortion of the chassis may result in distortion of the precise known alignment of the inertial sensors relative to the chassis. Since this uneven tightening of the swing bolts is not repeatable from one unit to the next, the alignment of the chassis relative to the aircraft support frame may vary as the chassis is replaced on the aircraft thereby contributing to non-repeatable and varying sensor alignment errors. This is so, since uneven tightening of the swing bolts can distort the chassis so that the front end of the chassis can twist slightly relative to the rear of the chassis. This potential distortion is dependent on the stiffness of the chassis and the difference in force between the left and right swing bolts. Any distortion of this type would result in a pitch alignment shift for a swing-bolt-mounted chassis which is oriented with the long axis of the chassis perpendicular to the long axis of the aircraft. Another consideration which can affect yaw alignment is that the threaded rod of the swing bolts will tend to stretch like a spring when the bolt is tightened causing yaw misalignment, and subsequent vibration may lead to further yaw misalignment.
Another disadvantage of the swing bolts inertial sensor mounting system is that they do not provide much resistance to sideways movement of the chassis on the shelf. This is so because the swing bolts are free to generally pivot from side to side. This too can cause inertial sensor alignment errors which may result in navigational computer output errors as the result of the chassis movement.